To control the release of a desired substance encapsulated within a molecular assembly such as liposomes, pH-responsive properties are commonly used. By way of example, liposomes based on phosphatidylethanolamine phospholipids are known (see, e.g., D. Papahadjopoulos et al., Biochemistry, 24 (1985) 3091-3098 (Non-patent Document 1), D. H. Thompson et al., Langmuir, 19 (2003) 6408-6415 (Non-patent Document 2)). This is based on the properties of phosphatidylethanolamine phospholipids which change their assembled structure in response to pH to thereby cause a change in their membrane permeability for a desired substance. However, liposomes containing dioleoylphosphatidylethanolamine (DOPE) can release the substance encapsulated therein only in an acidic environment (at pH 5 or less).
In addition, liposomes prepared from a mixture of anionic and cationic lipids are also known as pH-responsive liposomes (see G. Shi et al., Journal of Controlled Release 80 (2002) 309-319 (Non-patent Document 3)). However, this document fails to disclose that these liposomes release the substance encapsulated therein in a basic pH environment. Moreover, didecyldimethylammonium bromide (DDAB) having a quaternary amino group is used as a constituent member, and hence the liposomes are not intended to control the release of the encapsulated substance through ionization of amino groups. Further, there is a problem in that conventionally known cationic lipids are not suitable for administration to the human body because of their high cytotoxicity.
The inventors of the present invention have already found that liposomes which comprise, as a constituent lipid thereof, an amphiphilic molecule having a twitterionic functional group in its hydrophilic moiety hold a desired substance within their inner aqueous phase in a physiological pH environment and release the desired substance in an acidic pH environment (see JP 2007-210953 A (Patent Document 1) and WO2008/143339 (Patent Document 2)).
However, any pH-responsive molecular assembly has not yet been obtained, which holds a desired substance in an acidic pH environment and releases the desired substance in a basic pH environment.
In recent years, studies have been conducted to introduce a gene into cells by being conjugated with a cationic lipid alone or with a liposome containing the same. The inventors of the present invention have also developed a complex lipid suitable for such a purpose, which is low in cytotoxicity, is easy to synthesize and has a cationic functional group derived from an amino acid, and the inventors have also reported that such a complex lipid provides a formulation with high intracellular migration capability (see WO2006/118327 (Patent Document 3)).